U.S. patent number 8,937,332 [Application Number 13/982,667] was granted by the patent office on 2015-01-20 for wavelength converter for an led and led containing same.
This patent grant is currently assigned to OSRAM SYLVANIA Inc.. The grantee listed for this patent is David Hamby, Darshan Kundaliya, Kailash Mishra, Madis Raukas, Adam M. Scotch, Matthew Stough. Invention is credited to David Hamby, Darshan Kundaliya, Kailash Mishra, Madis Raukas, Adam M. Scotch, Matthew Stough.
United States Patent |
8,937,332 |
Kundaliya , et al. |
January 20, 2015 |
Wavelength converter for an LED and LED containing same
Abstract
A wavelength converter for an LED is described that comprises a
substrate of monocrystalline garnet having a cubic crystal
structure, a first lattice parameter and an oriented crystal face.
An epitaxial layer is formed directly on the oriented crystal face
of the substrate. The layer is comprised of a monocrystalline
garnet phosphor having a cubic crystal structure and a second
lattice parameter that is different from the first lattice
parameter wherein the difference between the first lattice
parameter and the second lattice parameter results in a lattice
mismatch within a range of .+-.15%. The strain induced in the
phosphor layer by the lattice mismatch shifts the emission of the
phosphor to longer wavelengths when a tensile strain is induced and
to shorter wavelengths when a compressive strain is induced.
Preferably, the wavelength converter is mounted on the light
emitting surface of a blue LED to produce an LED light source.
Inventors: |
Kundaliya; Darshan (Beverly,
MA), Raukas; Madis (Lexington, MA), Scotch; Adam M.
(Amesbury, MA), Hamby; David (Andover, MA), Mishra;
Kailash (North Chelmsford, MA), Stough; Matthew (Exeter,
NH) |
Applicant: |
Name |
City |
State |
Country |
Type |
Kundaliya; Darshan
Raukas; Madis
Scotch; Adam M.
Hamby; David
Mishra; Kailash
Stough; Matthew |
Beverly
Lexington
Amesbury
Andover
North Chelmsford
Exeter |
MA
MA
MA
MA
MA
NH |
US
US
US
US
US
US |
|
|
Assignee: |
OSRAM SYLVANIA Inc. (Danvers,
MA)
|
Family
ID: |
45812836 |
Appl.
No.: |
13/982,667 |
Filed: |
January 31, 2012 |
PCT
Filed: |
January 31, 2012 |
PCT No.: |
PCT/US2012/023222 |
371(c)(1),(2),(4) Date: |
July 30, 2013 |
PCT
Pub. No.: |
WO2012/106282 |
PCT
Pub. Date: |
August 09, 2012 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
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US 20130313603 A1 |
Nov 28, 2013 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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61439462 |
Feb 4, 2011 |
|
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Current U.S.
Class: |
257/98; 257/99;
117/108 |
Current CPC
Class: |
C30B
29/68 (20130101); C30B 23/025 (20130101); C30B
23/066 (20130101); H01L 33/502 (20130101); C09K
11/7774 (20130101); C09K 11/7721 (20130101); C09K
11/7701 (20130101); Y10T 428/24942 (20150115) |
Current International
Class: |
H01L
33/00 (20100101) |
Field of
Search: |
;257/98.99 ;117/108 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Kottaisamy et al., Color Tuning of Y3A15012:Ce phosphor and their
blend for white LEDS, Materials Research Bulletin 43 (2008)
1657-1663. cited by applicant .
May-Smith et al., Comparative growth study of garnet crystal films
fabricated by pulsed laser deposition, J. Crystal Growth 308 (2007)
382-391. cited by applicant .
Choe, Luminescence and compositional analysis of Y3Al5O12:Ce films
fabricated by pulsed-laser deposition, Mat. Res. Innovat. (2002)
6:236-241. cited by applicant .
Deng et al., Thin film rf magnetron sputtering of gadolinium-doped
yttrium aluminum garnet ultraviolet emitting materials, Optical
Materials 29 (2006) 183-191. cited by applicant .
Gundiah et al., Novel red phosphors based on vanadate garnets for
solid state lighting applications, Chem Phys. Lett. 455 (2008)
279-283. cited by applicant .
Setlur et al., Incorporation of Si4+-N3- into Ce3+--Doped Garnets
for Warm White LED Phosphors, Chem. Mater. (2008) A-G. cited by
applicant .
Chen et al., Effects of fluxes on the synthesis of
Ca3Sc2Si3O12:Ce3+ green phosphors for white light-emitting diodes,
Materials Science and Engineering B 166 (2010) 24-27. cited by
applicant .
Kucera et al., Defects in Ce-doped LuAG and YAG scintillation
layers grown by liquid phase epitaxy, Radiation Measurments 45
(2010) 449-452. cited by applicant .
Gill et al., Growth of crystalline Gd3Ga5O12 thin-film optical
waveguides by pulsed laser deposition, Materials Letters 25 (1995)
1-4. cited by applicant .
Zorenko, Luminescence of isoelectronic impurities and antisite
defects in garnets, Phys. Stat. Sol. (c) 2 No. 1 (2005) 275-379.
cited by applicant .
Kucera et al., Ce-doped YAG and LuAG Epitaxial Films for
Scintillation Detectors, IEEE Transactions on Nuclear Science vol.
55 No. 3 (2008) 1201-1205. cited by applicant .
Kucera et al., Growth and characterization of YAG and LuAG
epitaxial films for scintillation applications, J. Crystal Growth
312 (2010) 1538-1545. cited by applicant .
Zorenko et al., Single-crystalline films of Ce-doped YAG and LuAG
phosphors: advantages over bulk crystals analogues, J. Luminescence
114 (2005) 85-94. cited by applicant .
PCT Search Report for PCT/US2012/023222. cited by
applicant.
|
Primary Examiner: Dang; Phuc
Attorney, Agent or Firm: Clark; Robert F.
Parent Case Text
CROSS REFERENCES TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application
No. 61/439,462, filed Feb. 4, 2011.
Claims
We claim:
1. A wavelength converter for an LED, comprising: a substrate of
monocrystalline garnet having a cubic crystal structure, a first
lattice parameter and an oriented crystal face; and an epitaxial
layer formed directly on the oriented crystal face of the
substrate, the layer comprising a monocrystalline garnet phosphor
having a cubic crystal structure and a second lattice parameter
that is different from the first lattice parameter, the difference
between the first lattice parameter and the second lattice
parameter resulting in a lattice mismatch within a range of about
-3.1% to -15%.
2. The wavelength converter of claim 1 wherein the garnet phosphor
is a cerium-activated garnet phosphor having a composition
represented by a formula A.sub.3B.sub.5O.sub.12:Ce, wherein A is Y,
Sc, La, Gd, Lu, or Tb and B is Sc, Al or Ga.
3. The wavelength converter of claim 2 where the garnet phosphor is
selected from Y.sub.3Al.sub.5O.sub.12:Ce or
Lu.sub.3Al.sub.5O.sub.12:Ce.
4. The wavelength converter of claim 2 wherein the substrate is GGG
(111).
5. The wavelength converter of claim 2 wherein the phosphor
contains from about 0.001 to about 0.1 moles cerium per mole of
phosphor.
6. The wavelength converter of claim 2 wherein the phosphor
contains from about 0.005 to about 0.05 moles cerium per mole of
phosphor.
7. The wavelength converter of claim 2 wherein the garnet phosphor
is Lu.sub.3Al.sub.5O.sub.12:Ce and the substrate is GGG (111).
8. The wavelength converter of claim 3 wherein the substrate is GGG
(111).
9. The wavelength converter of claim 3 wherein the phosphor
contains from about 0.1 at.% Ce to about 10 at.% Ce.
10. The wavelength converter of claim 3 wherein the phosphor
contains from about 0.5 at.% Ce to about 5 at.% Ce.
11. The wavelength converter of claim 1 wherein the garnet phosphor
has a composition represented by a formula selected from: (i)
(A.sub.1A.sub.2')(B.sub.2B.sub.3')O.sub.12:Eu,Bi wherein A=Y, La,
Gd; A'=Na, K, Li; B=Mg, Ca, Sr, Ba; and B'=V, Ti, Sc, Nb, Zr; (ii)
A.sub.3B.sub.2B.sub.3'O.sub.12:Mg,Ce wherein A=Ca, Sr, Ba; B=Sc,
Ti, V, Nb; and B'=Si, Ge, Ga, Al, Sn, In; or (iii)
(AA').sub.3(BB').sub.5O.sub.12 wherein A=Y, La, Lu, Gd; A'=Mg, Ca,
Sr, Ba; and B or B', Al, Si, Ge.
12. The wavelength converter of claim 1 wherein the garnet phosphor
is represented by the formula
(A.sub.1A.sub.2')(B.sub.2B.sub.3')O.sub.12--:Eu,Bi wherein A is Y,
La, Gd; A' is Na, K, Li; B is Mg, Ca, Sr, Ba; and B' is V, Ti, Sc,
Nb, Zr or the garnet phosphor is represented by the formula
A.sub.3B.sub.2B.sub.3'O.sub.12:Mg,Ce wherein A is Ca, Sr, Ba; B is
Sc, Ti, V, Nb; and B' is Si, Ge, Ga, Al, Sn, In.
13. An LED light source comprising: an LED and a wavelength
converter mounted on a light emitting surface of the LED, the
wavelength converter converting at least a portion of the light
emitted by the LED into light having a longer wavelength, the
wavelength converter comprising: a substrate of monocrystalline
garnet having a cubic crystal structure, a first lattice parameter
and an oriented crystal face; and an epitaxial layer formed
directly on the oriented crystal face of the substrate, the layer
comprising a monocrystalline garnet phosphor having a cubic crystal
structure and a second lattice parameter that is different from the
first lattice parameter, the difference between the first lattice
parameter and the second lattice parameter resulting in a lattice
mismatch within a range of about -3.1% to -15%.
14. The LED light source of claim 13 wherein the garnet phosphor is
a cerium-activated garnet phosphor having a composition represented
by a formula A.sub.3B.sub.5O.sub.12:Ce, wherein A is Y, Sc, La, Gd,
Lu, or Tb and B is Sc, Al or Ga.
15. The LED light source of claim 13 wherein the garnet phosphor is
selected from Y.sub.3Al.sub.5O.sub.12:Ce or
Lu.sub.3Al.sub.5O.sub.12:Ce.
16. The LED light source of claim 13 wherein the garnet phosphor is
Lu.sub.3Al.sub.5O.sub.12:Ce and the substrate is GGG (111).
17. The LED light source of claim 13 wherein the garnet phosphor is
represented by the formula
(A.sub.1A.sub.2')(B.sub.2B.sub.3')O.sub.12:Eu,Bi wherein A is Y,
La, Gd; A' is Na, K, Li; B is Mg, Ca, Sr, Ba; and B' is V, Ti, Sc,
Nb, Zr or the garnet phosphor is represented by the formula
A.sub.3B.sub.2B.sub.3'O.sub.12:Mg,Ce wherein A is Ca, Sr, Ba; B is
Sc, Ti, V, Nb; and B' is Si, Ge, Ga, Al, Sn, In.
18. The LED light source of claim 13 wherein the LED emits light
having a wavelength in a range from 420 nm to 490 nm.
Description
TECHNICAL FIELD
This invention relates to wavelength converters for light emitting
diodes (LEDs). More particularly, this invention relates to thin
film converter elements for white light generation.
BACKGROUND OF THE INVENTION
The development in the 1990s of a high-brightness
blue-light-emitting LED made possible the introduction of
commercial high-efficiency, white-light-emitting LEDs that are
usable in general lighting applications. The realization of white
light from these monochromatic blue LEDs is achieved by employing
phosphors which convert at least a portion of the
shorter-wavelength blue light into longer green, yellow and red
wavelengths. One phosphor system of considerable interest for such
phosphor-conversion LEDs (pc-LEDs) is based on the family of
cerium-activated garnets represented by the general formula
A.sub.3B.sub.5O.sub.12:Ce, wherein A is Y, Sc, La, Gd, Lu, or Tb
and B is Sc, Al or Ga. These garnet-based phosphors have a cubic
lattice structure and absorb wavelengths in the range from 420 nm
to 490 nm which means that they are excitable by radiation from a
blue light source such as a blue LED. Other garnet phosphors such
as (Y, La, Gd)Na.sub.2Mg.sub.2V.sub.3O.sub.12:Eu, Bi and
Ca.sub.3Sc.sub.2Si.sub.3O.sub.12:Mg,Ce are also of interest for
pc-LEDs because of their red-light emissions which may be used to
improve the color rendering index (CRI) of white-emitting
pc-LEDs.
Of the garnet phosphors, cerium-activated yttrium aluminum garnet,
Y.sub.3Al.sub.5O.sub.12:Ce, (YAG:Ce), has seen widespread use in
commercial white-emitting pc-LEDs. The YAG:Ce phosphor has been
shown to be a very efficient converter for blue wavelengths
generating a broad intense yellow emission band centered at about
575 nm. This intense yellow emission and the remaining unconverted
blue light emitted by the LED combine to form a white light
emission.
One drawback to using the YAG:Ce phosphor by itself in a pc-LED is
that the white emission from the pc-LED has a high color
temperature and relatively low color rendering index (CRI). One way
to produce a warmer white light and increase CRI is to add a
red-emitting phosphor. However, phosphor mixtures tend to have
reduced efficacy because the phosphors interfere with one another
due to energy transfer via overlapping emission and absorption as
well as non-radiative processes. Another way to adjust the emission
of the pc-LED is to change the elemental composition of the
phosphor to increase output in the desired wavelength range.
Unfortunately, this can also affect the efficiency of the phosphor,
resulting in a lower efficacy LED.
Phosphor-conversion LEDs can also be used to produce single-color
LEDs by fully converting blue or UV light emitted by an LED into
another color such as green or red. This is desirable in some cases
because the pc-LED efficacy is greater than that of the comparable
monochromatic direct semiconductor LED. However, the range of
colors that can be produced by full conversion is similarly limited
by the ability to manipulate the composition of available
phosphors.
Thus it would be an advantage for both white and single-color
pc-LEDs to be able to adjust the emission colors of available
garnet-based phosphors without having to change their
composition.
SUMMARY OF THE INVENTION
It has been discovered that the emission wavelengths of garnet
phosphors, in particular YAG:Ce and LuAG:Ce (cerium-activated
lutetium aluminum garnet, Lu.sub.3Al.sub.5O.sub.12:Ce), can be
shifted in predictable ways by growing a monocrystalline phosphor
film on a monocrystalline substrate that has a slightly different
lattice parameter than the phosphor film. The strain induced in the
phosphor film by the lattice mismatch shifts the emission of the
phosphor to longer wavelengths (red shift) when a tensile strain is
induced and to shorter wavelengths (blue shift) when a compressive
strain is induced in the film growth direction. This effect is
believed to be a result of the modification in the ligand fields
around the activator ion (in this case Ce.sup.3+) that changes
electronic interaction between the activator and the ligands. Thus
it is possible to effect a change in the emission properties of
garnet-based phosphors without changing the phosphor's
composition.
More particularly, the garnet-based phosphor films are epitaxially
grown on an oriented crystal face of another monocrystalline cubic
garnet, e.g., undoped YAG (Y.sub.3Al.sub.5O.sub.12) or GGG
(Gd.sub.3Ga.sub.5O.sub.15). Preferably, pulsed laser deposition
(PLD) is used as a method to preserve the stoichiometry of the
phosphor films upon growth. The composition of growth substrate or
buffer layer is selected to increase/decrease the lattice mismatch
and hence influence the emission parameters (peak wavelength, band
width, etc.) in a desired way. The phosphor composition remains
unchanged.
In accordance with one aspect of the invention, there is provided a
wavelength converter for an LED, comprising a substrate of
monocrystalline garnet having a cubic crystal structure, a first
lattice parameter and an oriented crystal face; and an epitaxial
layer formed directly on the oriented crystal face of the
substrate, the layer comprising a monocrystalline garnet phosphor
having a cubic crystal structure and a second lattice parameter
that is different from the first lattice parameter, the difference
between the first lattice parameter and the second lattice
parameter resulting in a lattice mismatch within a range of
.+-.15%.
In accordance with another aspect of the invention, there is
provided a method of making a wavelength converter for an LED,
comprising forming an epitaxial layer of a monocrystalline garnet
phosphor on an oriented crystal face of a monocrystalline garnet
substrate, the monocrystalline garnet substrate having a cubic
structure and a first lattice parameter, and the monocrystalline
garnet phosphor having a cubic structure and a second lattice
parameter that is different from the first lattice parameter, the
difference between the first lattice parameter and the second
lattice parameter resulting in a lattice mismatch within a range of
.+-.15%.
In accordance with yet another aspect of the invention, there is
provided an LED light source comprising: an LED and a wavelength
converter mounted on a light emitting surface of the LED, the
wavelength converter converting at least a portion of the light
emitted by the LED into light having a longer wavelength, the
wavelength converter comprising: a substrate of monocrystalline
garnet having a cubic crystal structure, a first lattice parameter
and an oriented crystal face; and an epitaxial layer formed
directly on the oriented crystal face of the substrate, the layer
comprising a monocrystalline garnet phosphor having a cubic crystal
structure and a second lattice parameter that is different from the
first lattice parameter, the difference between the first lattice
parameter and the second lattice parameter resulting in a lattice
mismatch within a range of .+-.15%.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional illustration of a pc-LED according to
an embodiment of this invention.
FIG. 2 compares the photoluminescence under 450 nm excitation of
YAG:Ce films grown on YAG(100), YAG (111) and GGG (111)
substrates.
FIG. 3 is a .theta.-2.theta. x-ray diffraction (XRD) scan of a
YAG:Ce film on YAG (100).
FIG. 4 is an omega scan of the YAG:Ce film on YAG (100).
FIG. 5 shows .theta.-2.theta. XRD scans of YAG:Ce on YAG (111) and
GGG (111) substrates.
FIGS. 6 and 7 compare the photoluminescence under 450 nm excitation
of LuAG:Ce films grown on YAG(100), YAG (111) and GGG (111)
substrates.
FIG. 8 shows .theta.-2.theta. XRD scans of LuAG:Ce films on YAG
(111) and GGG (111) substrates.
DETAILED DESCRIPTION OF THE INVENTION
For a better understanding of the present invention, together with
other and further objects, advantages and capabilities thereof,
reference is made to the following disclosure and appended claims
taken in conjunction with the above-described drawings.
Epitaxy refers to the method of depositing a monocrystalline
(single crystal) film on an oriented monocrystalline substrate. The
deposited film is referred to as an epitaxial film or epitaxial
layer and has a definite crystal orientation with respect to the
lattice of the substrate on which it is grown. Epitaxial growth
requires that the atomic spacing, the lattice parameter (or lattice
constant), of the film material and the substrate material not
differ by more than a few percent.
Since the materials described herein have a cubic crystal
structure, the structure is definable by a single lattice parameter
(or lattice constant) "a." Consequently the lattice mismatch may be
defined in terms of how much the ratio of the lattice parameter of
the film to the lattice parameter of the substrate,
a.sub.film/a.sub.substrate, differs from unity,
a.sub.film/a.sub.substrate-1. The value of this mismatch may also
be expressed as a percentage,
(a.sub.film/a.sub.substrate-1).times.100%. Moreover, in terms of
the strain induced in the film, if a.sub.film>a.sub.substrate,
the film is tensioned in the direction of film growth and, if
a.sub.film<a.sub.substrate, the film is compressed in the
direction of film growth.
To increase the likelihood of obtaining a well-luminescing
epitaxial film, the substrate on which the film is grown should
have a similar crystal structure. If the lattice mismatch is too
large the film will not grow epitaxially and must be annealed in
order to become crystalline and luminescent.
The preferred garnet-based phosphors used in this invention may
represented by the general formula A.sub.3B.sub.5O.sub.12:Ce,
wherein A is Y, Sc, La, Gd, Lu, or Tb and B is Al, Ga or Sc. These
garnet-based phosphors have a cubic lattice structure and
preferably absorb wavelengths in the range from 420 nm to 490 nm.
Preferably, the concentration of cerium is from about 0.001 to
about 0.1 moles cerium per mole of phosphor. More preferably, the
concentration of cerium is from about 0.005 to about 0.05 moles
cerium per mole of phosphor. In the case of YAG:Ce or LuAG:Ce, the
concentration of cerium is preferably from about 0.1 atomic percent
Ce (at. % Ce) to about 10 at. % Ce, wherein atomic percent Ce (at.
% Ce) is defined as the number of cerium atoms divided by the total
number of cerium and yttrium (and/or lutetium) atoms expressed as a
percentage, {Ce/(Ce+Y,Lu)}*100%. More preferably, the concentration
of cerium is from about 0.5 at. % Ce to about 5 at. % Ce.
Other garnet phosphors include garnet phosphors having compositions
represented by the following formulas: (i)
(A.sub.1A.sub.2')(B.sub.2B.sub.3')O.sub.12:Eu,Bi wherein A=Y, La,
Gd; A'=Na, K, Li; B=Mg, Ca, Sr, Ba; and B'=V, Ti, Sc, Nb, Zr; (ii)
A.sub.3B.sub.2B.sub.3'O.sub.12:Mg,Ce wherein A=Ca, Sr, Ba; B=Sc,
Ti, V, Nb; and B'=Si, Ge, Ga, Al, Sn, In; or (iii)
(AA').sub.3(BB').sub.5O.sub.12 wherein A=Y, La, Lu, Gd; A'=Mg, Ca,
Sr, Ba; and B or B'=Al, Si, Ge.
A cross-sectional illustration of a pc-LED 15 according to an
embodiment of this invention is shown in FIG. 1. The wavelength
converter 25 is comprised of a monocrystalline thin film of a
garnet-based phosphor 10 that is grown epitaxially on an oriented
crystal face 30 of a substrate 20. The substrate 20 is comprised of
another cubic monocrystalline garnet having a similar, but not
identical, lattice parameter. The mismatch in the lattice
parameters between the film 10 and the substrate 20 create strain
in the film which shifts the emission peak of the phosphor film.
Since the film growth is epitaxial, the film has a definite crystal
orientation with respect to the crystal lattice of the substrate on
which it is grown. The substrate 20 is mounted to the top
light-emitting surface 35 of a blue-emitting LED 40. An InGaN LED
is particularly suitable as the blue-emitting LED. The blue light
emission from the LED has a wavelength in the range from 420 nm to
490 nm, and preferably 430 nm to 470 nm. UV- or near-UV-emitting
LEDs are also suitable pump sources for such pc-LEDs.
The blue light from the LED 40 having a wavelength A passes through
substrate 20, which is substantially transmissive with respect to
the blue light. The blue light is then at least partially absorbed
by the garnet-based phosphor film 10 and converted by the phosphor
film 10 into light having a longer wavelength B. The unabsorbed
blue light A and the light emitted by the phosphor B then produce a
combined emission A+B from the pc-LED which is perceived as white
light. (In the cases where full conversion is desired, only the
light emitted by the phosphor forms the emission from the
pc-LED.)
The thickness T.sub.1 of the film 10 is preferably from 200 nm to
20000 nm and the thickness T.sub.2 of the substrate is preferably
50 micrometers to 500 micrometers. The lattice mismatch between the
film and substrate should be within the range of .+-.15%,
preferably within the range of .+-.10%, and more preferably within
the range of .+-.5%. If the lattice mismatch, as defined
previously, is negative, i.e., the lattice parameter of the
substrate is greater than the lattice parameter of the film, then a
compressive strain is created in the phosphor film which shifts the
emission to shorter wavelengths. If the lattice mismatch is
positive, i.e., the lattice parameter of the film is greater than
the lattice parameter of the substrate, a tensile strain is created
in the phosphor film which shifts the emission to longer
wavelengths. Theoretically, this can be explained as a change in
the crystal field splitting parameter 10Dq which is the energy
difference between the low-energy three-fold degenerate electronic
state, t.sub.2g, and the high-energy two-fold degenerate state,
e.sub.g, of the trivalent cerium (Ce.sup.3+) The compressive strain
with its resultant shift towards shorter wavelengths (blue-shift)
in garnets may be viewed due to a reduction of 10Dq. Conversely,
the phosphor emission can be shifted to longer wavelengths
(red-shifted) if the strain compels 10Dq to increase. By modifying
the level of strain in the film, one can adjust the spectral
emission to the desired output. The amount of strain in the film
can also be further controlled by adjusting the thickness of
phosphor film due to a gradual relaxation of the strain as the film
thickness increases.
EXAMPLES
Epitaxial YAG:Ce Films
Strain-Induced Engineering for White pc-LEDs
Thin films of YAG:Ce were grown on YAG (100), YAG (111) and GGG
(111) substrates under identical growth parameters using pulsed
laser deposition (PLD). The PLD growth parameters are given in
Table 1. The films grew epitaxially and the as-deposited films
exhibited the expected photoluminescence (PL). This is in contrast
to similar films grown on c-Al.sub.2O.sub.3 and fused silica
(quartz) substrates where a post-annealing treatment was necessary
to observe photoluminescence. As shown in Table 1, the emission
bands for the YAG:Ce films grown on YAG (111) and GGG (111) were
red and blue shifted, respectively, from that of the films grown on
YAG (100). This trend was observed irrespective of the density of
the target.
TABLE-US-00001 TABLE 1 Growth Emission Laser temperature Thickness
wavelength Substrate energy (.degree. C.) (nm) (nm) YAG-100 200 mJ
1050 950 592 GGG-111 200 mJ 1050 880 569 YAG-111 200 mJ 1050 804
596
FIG. 2. shows the PL under 450 nm excitation of YAG:Ce (4at. % Ce)
films grown on YAG (100), YAG (111) and GGG (111) substrates. The
primary reason for the shifting emission appears to be the lattice
mismatch between the films and substrates. The lattice parameter,
a, is 12.01 .ANG. for YAG and 12.382 .ANG. for GGG with a cubic
structure. When YAG:Ce is grown on GGG it experiences compressive
strain with a lattice mismatch of about -3.1% resulting in a
blue-shifted emission at 569 nm. On the other hand, when using YAG
(111) or YAG (100) substrates, the lattice mismatch to Ce:YAG is
<1% (larger Ce.sup.3+ (ionic radius 115 pm) replaces Y.sup.3+
(ionic radius 104 pm)). This induces tensile strain and hence red
shift. These results demonstrate that one can selectively achieve
the desired spectral output by selecting the appropriate substrate.
When placed on blue LEDs, the cumulative output of the PL from the
film and the pump LED will define each time a different color
point.
X-ray diffraction studies were carried out on several thin films
grown on the garnet substrates YAG (100), YAG (111) and GGG (111).
As expected, films were epitaxially oriented as 100 and 111 planes
when grown on 100 and 111 oriented garnet substrates, respectively.
It has been also noted that crystalline quality of the film is
better when grown on YAG (100) substrate. FIG. 3 shows
.theta.-2.theta. XRD scan of YAG:Ce on YAG (100). The dotted line
is the diffraction peak of YAG (100) substrate whereas the solid
line represents the same from the YAG:Ce film on this substrate. It
can be seen that only peaks for the expected epitaxial (400) and
(800) orientations have been detected. FIG. 4 is an omega scan for
determination of the degree of crystallinity by measuring full
width at half maximum (FWHM) of the film peak. It turns out that
the FWHM is approx. 0.0572.degree. suggesting a high quality
monocrystalline growth.
FIG. 5 shows .theta.-2.theta. XRD scan of YAG:Ce thin films on YAG
(111) and GGG (111) substrates. On YAG(111), the YAG:Ce film
experiences a clear tensile strain that makes the `d` spacing
larger (corresponding to lower 20 values) and on GGG(111) it
experiences a compressive strain that makes the `d` spacing smaller
(corresponding to higher 2.theta. values).
Epitaxial LuAG:Ce Films
Strain-Induced Engineering for Single-Color Green pc-LEDs
Epitaxial thin films of Lu.sub.3Al.sub.5O.sub.12:Ce (LuAG:Ce) were
deposited on monocrystalline YAG (111), YAG (100) and GGG (111)
substrates by PLD. The PLD growth parameters for the LuAG:Ce (0.5
at. % Ce) films are given in Table 2. The lattice parameter of LuAG
is 11.91 .ANG. and that of GGG is 12.382 .ANG., both with cubic
structure. Hence, the lattice mismatch for depositing LuAG on GGG
is about -3.8% compared to about -0.8% on YAG (111). The greater
mismatch in lattice parameters manifests itself in a blue-shifted
emission for LuAG:Ce on GGG (111) compared to thin films of LuAG:Ce
grown on YAG substrates.
TABLE-US-00002 TABLE 2 Laser Growth Emission energy temperature
Thickness wavelength Gas environment Sample (mJ) (.degree. C.) (nm)
(nm) During growth During cooling (a) YAG-100 300 1050 593 -- 3
mTorr O.sub.2 300 Torr 3% H.sub.2 + N.sub.2 balance (b) YAG-111 300
1050 501 513/562 3 mTorr O.sub.2 300 Torr 3% H.sub.2 + N.sub.2
balance (c) GGG-111 300 1050 603 507/562 3 mTorr O.sub.2 300 Torr
3% H.sub.2 + N.sub.2 balance (d) YAG-111 300 1050 391 511/553 3
mTorr O.sub.2 3 mTorr O.sub.2 (e) GGG-111 300 1050 377 505/549 3
mTorr O.sub.2 3 mTorr O.sub.2
The PL spectra a (.lamda..sub.excitation=450 nm) of the LuAG:Ce
(0.5 at % Ce) films grown on YAG(100) YAG (111) and GGG (111) are
shown in FIGS. 6 and 7. The broad emission band is observed to
consist of two Gaussian components. In both cases, the first
Gaussian component centered at 562 nm remains virtually independent
of the substrate while the second Gaussian component shifts its
position depending on the substrate used. The blue shift of the
second Gaussian component in the case of the LuAG:Ce film grown on
the GGG (111) substrate amounts to 6 nm (from 513 nm to 507 nm).
This blue shift can be correlated with the creation of compressive
strain on the Ce.sup.3+ luminescence centers compared to the
activator environment in a typical (bulk or unstrained) LuAG.
FIG. 8 shows .theta.-2.theta. XRD scan of LuAG:Ce on YAG (111) and
GGG (111) substrates. It can be seen that only peaks for the
expected epitaxial orientations have been detected. The substrate
peaks are denoted by `S` and the film peaks are denoted by `F`. It
can be clearly seen from the film peak position that LuAG:Ce has
different strain effects when grown on YAG or on GGG
substrates.
While there have been shown and described what are at present
considered to be preferred embodiments of the invention, it will be
apparent to those skilled in the art that various changes and
modifications can be made herein without departing from the scope
of the invention as defined by the appended claims.
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